Part of the discussion is that user interface is now a conduit for branding, probably the major conduit for software focused companies. You can't have consistent UI branding across devices unless you replace the UI to suit your branding. So rather than user interface being refined over the years into a consistent usable tool for each platform, by people specifically interested in UI and the paradigms of that platform, it's re-implemented year-over-year, with UX playing second fiddle to branding and design.